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OpAmp with low common mode distortion

Category: Datasheet/Specs

We are looking for an op-amp to replace the input stage of a data acquisition instrument. Currently the front-end is an AD8510, configured as a non-inverting buffer. This frontend shall now be expanded with a 10:1 resistor divider to allow measuring higher input voltages. This attenuator however increases the source impedance as seen by the AD8510 to ~100kohms. We noticed a significant increase in THD:

A 10Vpk 1kHz sinewave connected without the attenuator (low source resistance, about 5k ohms) gives -105dB THD for the complete system, but the same signal connected via the attenuator results in -65dB THD. I assume this is caused by modulation of the op-amp input capacitance, which has no ill effect as long as the source resistance is small, but not so with a high source resistance.

Can you recommend a replacement for the AD8510 with low common mode distortion? The system constraints are:

single device
MSOP8 package
+/-15V operation
CM input range V+ - 2V, V- +2V or better
low voltage and current noise
low voltage and bias current offset
supply current less than 3mA

Many thanks, AK

Top Replies

    •  Analog Employees 
    •  Super User 
    Jun 6, 2023 +1 verified

    Hi  ,

    We saw an alternative for your AD8510 that you could use in your system which is the ADA4610. However, the ADA4610-1 is only available in an 8-lead SOIC package and in a 5-lead SOT-23 package…

  • Hello AK,

    May I know the application of your circuit? Also, can you send me your schematic? 

    Thank you!

    Regards, 

    Mae

  • Hello Mae,

    The application is a data acquisition frontend to measure sensors with an output voltage up to +/-12V with a bandwidth from DC up to 50kHz. The AD8510 is followed by a differential ADC driver (ADA7490) and an AD7768 ADC. The AD8510 operates as a run-of-the-mill non-inverting amplifier, either as a voltage follower or configured to a gain of 10.The input resistance should be as high as possible, minimum is 250K.

    There is a demand to measure higher input voltages which requires a passive pre-divider. First tests with a 200K - 220K divider (+/- a 24V input range) resulted in too high THD figures.

    Another option is implementing a 20:1 divider (380K - 20K) and using the x10 gain. This should significantly reduce the CM distortions but adds 20dB noise ...  So the preferred solution is a compatible op-amp with a low CM distortion figure.

    Best Regards, bdolf

  • Hi  ,

    We saw an alternative for your AD8510 that you could use in your system which is the ADA4610. However, the ADA4610-1 is only available in an 8-lead SOIC package and in a 5-lead SOT-23 package. The ADA4610-2 is the one available in 8-lead MSOP, 8-lead SOIC, and 8-lead LFCSP packages. 

    Hope this helps!

    Thank you!

    Best regards,

    Mae